A problem of noise suppression (i.e., improving Optical Signal to Noise Ratio OSNR) in long communication lines is a very complex problem, which always required great efforts and high investments at the transmitting end, at nodes of the communication line and at the receiving end. Today, there are no means that enable a network operator or designer to improve OSNR using passive components (i.e., those not comprising electro-optical schemes). For modern optical communication lines, which operate at very high bitrates, the problem becomes critical; the only way to improve OSNR in such systems is to regenerate the optical signal. A conventional signal regeneration assembly comprises expensive active blocks such as an optical receiver (Rx), an Opto-Elecro-Optic regeneration block and an optical transmitter (Tx). The regeneration cost today is approximately $10 k for an optical channel transmitting signals at 10 Gb/s, the regeneration cost jumps to $30˜50 K for the bit rate 40 Gb/s per channel. Moreover, new high bit rate systems operating at 100 Gb/s are being presently developed and are on their way to the market. In some cases the regeneration is impossible for several reasons (for example in underwater communication links and the like). For such cases, there is no solution to efficiently overcome the problem by now.
Second Harmonic Generation devices (comprising SHG or non-linear crystals) are known in the prior art, and are usually utilized as components in lasers. The SHG property of converting an optical beam having a fundamental wavelength into a light beam having a twice-shorter (second harmonic) wavelength, where frequency noise of the source is reduced, is widely utilized for producing coherent light in lasers.
US 2006045148 describes a low noise, intra-cavity frequency doubling micro chip laser utilizing a non-linear crystal.
SHG devices are also used in combination with lasers, for example US2003/0007205 (U.S. Pat. No. 6,785,471) describes an optical sampling technique. The probe pulse source is frequency-doubled e.g., using a frequency doubler such as a nonlinear PPLN (Periodically Poled Lithium Niobate) crystal, to obtain an intermediate second harmonic, which may be filtered with a 780 nm bandpass filter to eliminate at least the source frequency noise background. The filtered intermediate second harmonic is then mixed with the user input signal by utilizing an optional polarizing beam splitter and a dichroic beam splitter. The mixed signal is sent to a sum frequency generating (SFG) nonlinear crystal, e.g., a PPLN crystal, where the resulting frequency is near the third harmonic. The output from the SFG PPLN crystal may be filtered using a bandpass 515 nm filter to remove unwanted wavelengths, and then processed to measure/sense the near third harmonic content using a photomultiplier tube (PMT).
US2003026573A describes yet another, specific design of a waveguide device incorporating an SHG element, the waveguide is intended to be used with a laser and enables reducing the following types of noise: low frequency interference noise and mode hopping noise. The low frequency interference noise occurs when light emitted from a so-called SHG blue light source (formed by the SHG element) is reflected and returns to the exit end face of the SHG blue light source to cause interference in the optical system outside the SHG blue light source. The mode hopping noise results from the inside of the laser.
To the best of the Applicant's knowledge, SHG elements have always been used for reducing specific types of noise created in the transmitter/laser systems, so as to obtain maximally clean coherent signals suitable, for example, for transmission via communication lines over long distances.